In recent years, electronic devices are becoming multifunctional, smaller in size and lighter in weight. In particular, there is a strong demand for smaller, lighter and thinner mobile electronic devices, such as mobile phones and digital still cameras, because these devices are carried by people. As a mounting technique that satisfies this demand, mounting structures have been developed in which a primary electronic component, such as a semiconductor element, is mounted on a multilayer circuit board.
A typical mounting structure includes a structure in which an LGA (land grid array) package, which is one type of semiconductor packages, is mounted on a multilayer circuit board. In an LGA package, planar electrodes called lands are arranged in a matrix on the undersurface of an interposer board onto which a semiconductor chip is fixed.
FIG. 11 shows an external view of a conventional mounting structure MSp in which a flat LGA package is mounted on the mounting surface of a circuit board. FIG. 12 shows part of a cross section of the conventional mounting structure MSp, taken along the line XII-XII of FIG. 11. Underfill is injected into the gap between a circuit board 1p and the LGA package 2 in order to reinforce solder connection, but FIGS. 11 and 12 show a state before the underfill is injected.
Lands 21 are formed in a matrix on the undersurface of the LGA package 2. On the mounting surface Sm of the circuit board 1p, terminals 11 arranged in a matrix that correspond to the lands 21 of the LGA package 2 and a wiring pattern (not shown) connected to the terminals 11 are formed. Although not shown, three-dimensional multilayer wiring having through holes is formed inside the circuit board 1p. 
A process for manufacturing the conventional mounting structure MSp will be described next briefly. First, a solder paste containing flux is applied onto the terminals 11 of the circuit board 1p by screen printing or the like. Then, the LGA package 2 is placed on the circuit board 1p with the side on which the lands 21 are formed facing down.
At this time, the terminals 11 of the circuit board 1p and the lands 21 of the LGA package 2, provided at a predetermined pitch, are positioned so as to face each other with solder paste therebetween. Subsequently, the solder paste is heated in a reflow oven to join the terminals 11 and the lands 21 with solder 3.
Next, underfill is injected into the gap between the circuit board 1p and the LGA package 2c thus connected. As shown in FIG. 11, a solution containing underfill 4 is dripped on the boundary between the circuit board 1p and the LGA package 2. Then, the underfill 4 permeates between the circuit board 1p and the LGA package 2 by capillarity, and fills the gap except for the solder joints 3.
Subsequently, the underfill 4 is heated and cured to reinforce the connection achieved with the solder between the circuit board 1p and the LGA package 2. In this manner, the mounting structure MSp is obtained.
With the above-described conventional mounting structure MSp, a case may arise where the permeation of the underfill 4 is hindered by residual flux seeped out from the solder joints 3 when injecting the underfill 4 into the gap between the LGA package 2 and the circuit board 1p, and a sufficient connecting strength between the circuit board 1p and the LGA package 2 cannot be obtained.
Specifically, because the gap (standoff) between the circuit board 1p and the LGA package 2 is as extremely small as about 100 μm, liquid flux seeped out from the solder paste may spread to the space in the proximity of the terminal 11 by capillarity during a reflow process, and the space between adjacent terminals 11 may be filled with the flux. After evaporation of solvent, the solid content of the flux that has spread to the mounting surface Sm of the circuit board 1p remains as residual flux. The residual flux left in the gap between the circuit board 1p and the LGA package 2 as described above hampers the connection between the circuit board 1p and the LGA package 2 with the underfill 4.
The state of flux 5 when the mounting structure MSp is in a reflow process will now be described in detail with reference to FIG. 13. Similarly to FIG. 12, (A) to (E) in FIG. 13 show partial enlarged views of a cross section of the mounting structure MSp, which respectively correspond to soldering steps in a reflow process.
The solvent contained in the flux eventually evaporates, so that only residual flux remains on the circuit board 1p. Ordinarily, an alcohol is used as the solvent, but since the content is several percent, there is almost no change in the flux shape between before and after evaporation of the solvent. Hereinafter, flux in a liquid state is referred to as flux 5, and flux in a solid state after the solvent has evaporated is referred to as a residual flux 5d to distinguish them from each other.
When temperature is increased to the melting temperature of the solder joints 3, as shown in (A) of FIG. 13, flux 5 separated from the solder joints 3 as a result of the temperature rise seeps out. If the amount of the seeped flux 5 is small, the flux 5 travels over the surface of the solder joints 3 and reaches the terminal 11 of the circuit board 1p, but it does not reach the circuit board 1p. 
If the amount of the flux 5 seeped out from the solder joints 3 is large, as shown in (B) of FIG. 13, the flux 5 reaches the circuit board 1p and spreads around the terminal 11.
The flux 5 seeped out from the adjacent solder joints 3 eventually merges on the circuit board 1p, forming a kind of coating that partially covers the surface of the circuit board 1p as shown in (C) of FIG. 13.
If more flux 5 seeps out from the solder joints 3, as shown in (D) of FIG. 13, the coating increases in area and thickness, and piles up in the space between the adjacent solder joints 3.
The coating continues to increase in thickness, and finally, as shown in FIG. 13E, it fills locally between the LGA package 2 and the circuit board 1p. 
At the time when the reflow process is finished, flux 5 is left around the solder joints 3 in any one of the states shown in (A) to (E) of FIG. 13. Hereinafter, the states of flux shown in FIGS. 13A to 13E are referred to as FR1, FR2, FR3, FR4 and FR5, respectively, as indicated on the right side of the diagrams, in order to distinguish them from each other.
As described above, before underfill 4 is injected, residual flux 5d exists inside the mounting structure MSp in any one of the states FR1 to FR5. The effect of underfill 4 for reinforcing the mounting structure MSp will be described below briefly for each of the states FR1 to FR5 of the residual flux 5d. 
In the case of FR1 (see (A) in FIG. 13), the residual flux 5d stays at the terminal 11 and has not reached the circuit board 1p. Accordingly, the circuit board 1p and the LGA package 2 will be bonded completely with the underfill 4. After being cured, the underfill 4 adheres to the surface of the solder joint 3, and retains the shape of the solder joint 3. The greatest reinforcing effect of the underfill 4 on the mounting structure MSp is obtained in this case.
In the case of FR2 (see (B) in FIG. 13), the residual flux 5d has seeped out from the terminal 11 onto the circuit board 1p, but there is a region in which the residual flux 5d does not exist in the space between the adjacent solder joints 3 (terminals 11). In this case, the underfill 4 can adhere only partially to the circuit board 1p, but it can adhere completely to the LGA package 2. Furthermore, the function for retaining the shape of the solder joint 3 is also achieved. Accordingly, in the case of FR2, the circuit board 1p and the LGA package 2 are bonded relatively strongly although the strength is lower than that of the case of FR1. Thus, the effect of underfill 4 for reinforcing the mounting structure MSp is the greatest after FR1.
In the case of FR3 (see (C) in FIG. 13), the surface of the circuit board 1p is covered with residual flux 5d in the form of a coating. Accordingly, the underfill 4 can adhere to the LGA package 2, but it cannot adhere to the circuit board 1p. The function of underfill 4 for retaining the shape of the solder joint 3 itself is effective. However, it is difficult to say that the effect of underfill 4 for reinforcing the mounting structure MSp is effective because the underfill 4 does not adhere to the circuit board 1p, although the effectiveness is the greatest after FR2.
In the case of FR4 (see (D) in FIG. 13), the situation is similar to that of the case of FR3, but the function of underfill 4 for retaining the shape of the solder joint 3 cannot be achieved. Accordingly, the effect of underfill 4 for reinforcing the mounting structure MSp is lower than that of the case of FR3.
In the case of FR5 (see (E) in FIG. 13), the gap between the circuit board 1p and the LGA package 2 is completely filled with the residual flux 5d, so that the underfill 4 cannot enter. That is, the circuit board 1p and the LGA package 2 will not be bonded by the underfill 4. Accordingly, the effect of underfill 4 for reinforcing the mounting structure MSp is not obtained.
FIG. 14 shows an example of the mounting structure MSp obtained by curing after allowing underfill 4 to permeate when residual flux 5d exists in any one of the states FR1 to FR5, which is viewed in the direction indicated by the arrow A of FIG. 11 after the LGA package 2 has been removed from the mounting structure MSp shown in FIG. 11. Although wires are attached to some of the terminals 11, the wires are not shown in FIG. 14 for the sake of clarity.
In this example, residual flux 5d has spread in three regions (R1, R2, R3) on the mounting surface Sm of the circuit board 1p. The flux spread region R1 extends over ten solder joints 3 (terminals 11), and the flux spread regions R2 and R3 extend over three solder joints 3 (terminals 11).
FIG. 15 shows the cross section of the mounting structure MSp of FIG. 14 in which filling and curing of underfill 4 has been performed, taken along the XV-XV line of FIG. 14. In FIG. 15, the region shown on the far left that is outside the flux spread region R1 is a region in which residual flux 5d exists in the state FR1, or in other words, a region in which residual flux 5d does not exist except the terminal 11. Accordingly, the circuit board 1p and the LGA package 2 are bonded by the underfill 4, and the shape of the solder joint 3 is retained by the underfill 4. Such a region is referred to as a residual flux region P(FR1).
Around solder joints 3 located at the ends of the flux spread region R1, residual flux 5d exists in the FR2 state. In other words, a partial coating of residual flux 5d is formed. The underfill 4 adheres partially to the circuit board 1p and completely to the LGA package 2, and retains the shape of the solder joint 3. Such a region is referred to as a residual flux region P(FR2).
Between a solder joint 3 located at an end of the flux spread region R1 and a solder joint 3 adjacent thereto, residual flux 5 exists in the FR3 state. In other words, the circuit board 1p is covered completely by a relatively thin coating of residual flux 5d. The underfill 4 adheres completely to the LGA package 2, but it cannot adhere to the circuit board 1p. The underfill 4 retains the shape of the solder joint 3. Such a region is referred to as a residual flux region P(FR3).
Between a solder joint 3 located at the center portion of the flux spread region R1 and a solder joint 3 adjacent thereto, residual flux 5d exists in the FR4 state. In other words, the circuit board 1p is covered with a relatively thick coating of residual flux 5d. The underfill 4 adheres completely to the LGA package 2, but it cannot adhere to the circuit board 1p. The underfill 4 can retain only part of the shape of the solder joint 3. Such a region is referred to as a residual flux region P(FR4).
Between adjacent solder joints 3 located at the center portion of the flux spread region R1, residual flux 5d exists in the FR5 state. In other words, the space between the circuit board 1p and the LGA package 2 is filled with residual flux 5d. Accordingly, the underfill 4 cannot adhere to the circuit board 1p and the LGA package 2, and it cannot retain the shape of the solder joints 3. Such a region is referred to as a residual flux region P(FR5).
If portions, such as the region P(FR5), that are clogged with residual flux dot the circuit board 1p, the underfill 4, when permeating by capillarity, spreads in multiple directions so as to avoid the portions clogged with residual flux. As a result, although not shown in the example of FIG. 14, air is trapped in the gap in some places, creating air bubbles. The trapped air (air bubbles) expands when heating and curing the underfill 4, and forces out the underfill 4. Consequently, the contact areas between the underfill 4 and the circuit board 1p and between the underfill and the LGA package 2 are reduced, and therefore a sufficient connection strength cannot be secured.
As described above, if residual flux 5d separated from the solder exists between the terminals 11 of the circuit board 1p, filling of the underfill 4 is prevented by the residual flux 5d, and therefore a sufficient connection strength cannot be secured between the circuit board 1p and the LGA package 2. In addition, the function of underfill 4 for retaining the shape of the solder joints 3 cannot be exhibited sufficiently.
To solve the above problems, a method is conceivable in which the residual flux is removed by washing the mounting structure MSp after the soldering step in the reflow process is finished. However, the inclusion of a residual flux washing step makes the manufacturing process complicated, and substances produced during washing of the residual flux can cause environmental damage. It is therefore desirable to perform soldering without washing residual flux.
As a method for injecting underfill between a circuit board and a package without washing residual flux while securing a sufficient connection strength, Japanese Laid-Open Patent Publication No. 2006-294835 proposes to provide air vents that pass through the circuit board.
However, the flux separated from the solder is liquid, and thus the liquid flux flows from the flux source (solder joints) to the air vents. That is, the flux spreads over the circuit board, and after having passed therethrough, residual flux is left.
Furthermore, in order to suppress residual flux, it is necessary to increase the number of air vents as well as the diameter of the air vents. Even if the ejection of liquid flux is facilitated, however, the underfill is ejected from the air vents to the outside in the underfill injection step. As a result, an amount of underfill sufficient to reinforce the connection strength between the circuit board and the electronic component may not be obtained.